3Ocean Research Institute and Department of Earth and Planetary Sciences, University of Tokyo,
1-15-1 Minamidai, Nakanoku, Tokyo
164-8639, JAPAN

4Institute for Research on Earth Evolution, JAMSTEC, Yokosuka, Japan.

So far, the
most complete and accurate sea-level record that encompassed the period between
the Last Glacial Maximum and the present day is based on cores drilled offshore
from the Barbados
coral reef [1, 2]. That record suggests a non-monotonous sea-level rise
punctuated by two dramatic accelerations, the MWP1-A and MWP1-B events,
centered at ~14,000 and ~11,300 cal. yr BP respectively [3, 4]. However, the occurrence, the hemispheric origin as well
as the exact relationship between those events and the global climatic
evolution remain enigmatic and controversial. The recent IODP Expedition 310
"Tahiti Sea Level" offers a unique opportunity to extend the previous
Tahiticoral reef record that documented the deglacial sea-level rise over the
last 13.8 ka [5, 6]. Located at a considerable distance from the major
former ice sheets and characterized by slow and regular subsidence rates, Tahiti provides an ideal setting to constrain MWP events
that are thought to have punctuated the last deglaciation. The offshore coring
operations carried out during the Expedition 310 recovered more than 400 m of
post-glacial reef material, ranging from 122 to 40 m below modern sea level [7, 8].

More than 60 U-Th ages were obtained on various types of corals that are indicative of
a range of modern reef environments, from the reef crest to the reef slope. Together with previous on-shore data [5, 6], this new data set extend the previous Tahiti
record to the last 16 ka and allow to document the sea-level rise during the
previously defined key period of the MWP-1A [4]. Our results confirm the
occurrence of an acceleration of the sea-level rise during this period.
However, the timing and duration of this event are significantly different to those
of the MWP-1A as defined in Barbados
[3]. These new results allow us to revisit the relationship between the MWP-1A
and the climate history of the last deglaciation. We will discuss also their
implications in terms of the potential sources of the ice that generated the
MWP-1A.

We also take advantages of the good preservation of the post-glacial
coral material collected during the IODP Expedition 310 to investigate
potential variations with time of the seawater uranium-isotope ratio. The
history of seawater (234U/238U)SW, as inferred
from compilation of d234U values in U-series dated corals, suggested
long-term to abrupt shifts in the (234U/238U)SW
related to changes in the oceanic budget due to fluctuations of sea-levels or
global weathering rates [9-13]. Before U-Th datings, rigorous mineralogical and
isotopic screening criteria have been applied in order to preclude any
post-mortem diagenetic alteration of the aragonitic coral skeleton. We checked the absence of secondary calcite content in
aragonite skeleton by using X-ray diffraction precisely calibrated by using
gravimetric standards [14]. In particular,
we made a significant effort to improve detection and quantification of very
small amount of secondary calcite [14]. Only coral samples with less than 1% calcite content were
analyzed.U-Th analyses were performed using a VG-54 thermo-ionisation
mass spectrometer equipped with a 30-cm electrostatic analyzer and a
pulse-counting Daly detector. Calculated initial (234U/238U)0
values of post-glacial samples analysed in this study fall within 2-3‰ of the
most recent determinations of the uranium isotopic composition of modern corals
and present-day seawater [15-17].

The analytical reproducibility achieved in the course of the study for
the ratio (234U/238U) is about 0.8‰ (2s). All samples
fall within the isotopic range adopted by Hughen et al. [18] for the interval 0-17 ka ((234U/238U)0
= 1.1452±0.0048, 2s) as a strict isotopic screening criteria. However, for
coeval coral samples, the (234U/238U)0 values
are highly consistent within our internal reproducibility. The high consistency
of the new Tahiti dataset highlights the remarkable preservation of samples
recovered in the Tahiti offshore reef cores.
The clustering of the (234U/238U)0 values
substantially narrows the uncertainties in marine signature for the period 11 –
16 ka and allow to portray the (234U/238U) marine
signature fluctuation in an unprecedented way. We will compare our results with
previous dataset (Vanuatu, Papua New Guinea and Barbados) that encompass this
period [3, 17, 19] in order to elucidate subtle fluctuations of the
marine signature during the last deglaciation.

15.Delanghe,
D., E. Bard, and B. Hamelin, New TIMS constraints on the uranium-238 and
uranium-234 in seawaters from the main ocean basins and the Mediterranean Sea.
Marine Chemistry, 2002. 80(1): p. 79-93.